Pistons which reciprocate within a cylinder bore are well known in the art and have been long used for pumps, hydraulic drives, engines, pneumatic drives and other hydraulic or pneumatic machines delivering different fluids at increased pressure. When the piston is moving within the cylinder, a piston seal assembly is employed for dynamic sealing to prevent fluid from leaking between the piston and the cylinder walls.
Piston seals are subjected to substantive wear due to the piston's movement within the cylinder bore and sometimes they fail due to excessive wear and/or due to the stress caused by the pressure exerted on the seal by the fluid being compressed. Such operating conditions can be even more challenging in the case of high pressure reciprocating pumps and/or in the case of pumps handling cryogenic fluids.
An example of such a reciprocating pump is a pump used to deliver a liquefied gaseous fuel from a cryogenic vessel to a gaseous-fuelled internal combustion engine. Such pumps can handle fluids at relatively high pressures, around 4600 psi, and at low temperatures, for example at around −130 degrees Celsius and lower. It is difficult to provide an effective seal for the piston of such a pump especially because the material of the seal has to withstand the low temperatures of the pumped fluid and the high compression stresses acting on the seal. Also, the different contraction coefficients of the materials from which the pump components and the seals are made can result in gaps being formed between the piston seals and the cylinder wall allowing fluid to leak from the compression chamber past the piston seal to the low pressure side of the pump.
For all the reciprocating pumps in general, seal wear due to the reciprocating movement of the piston is a problem that needs to be addressed. In the past, split seals have been used to address the seal wear problem. Split seals have the shape of a ring having a cut which allows it to be installed on the piston and more importantly it allows the seal to expand to compensate for the seal wear. For maintaining the contact between the seal and the pump cylinder wall a seal energizer is provided for supporting the seal and pushing it towards the cylinder wall. The seal energizer can be a metal ring that is positioned underneath the seal and forms together with the seal a piston seal assembly that is placed in a piston groove. The seal energizer is split transversally such that there is a gap between the free ends of the ring which provides for an easier installation of the seal energizer in the piston groove and provides more elastic force for pushing the seal towards the cylinder wall when the seal assembly is installed in the piston groove.
Generally there is a clearance gap between the seal energizer and the lateral walls of the piston groove where the seal assembly is positioned due to manufacturing tolerances. The dimensions of the gap can vary, but it can be in some cases larger than 0.15 mm. The size of the gap between the seal energizer and the piston groove can be controlled only through tight manufacturing tolerances and such gap represents a potential seal leak path which can affect the life span of the seal. With the known construction of seal energizers the seal leak path area can vary depending on the manufacturing tolerances of the seal energizer and of the piston groove and depending on the pump's operating conditions.
Furthermore, if several seal assemblies, each comprising a seal and a seal energizer, are mounted in series for sealing the piston as it reciprocates within the pump cylinder, it has been observed that some seals show substantially more wear than others with at least one random seal failing after some time. This can be explained by the fact that the pressure drop across each seal varies. The pressure drop across each seal depends on the size of the leak path area through the seal, and seals with the smallest leak path area experience the greatest pressure drop due to the highest resistance to fluid pressure and will wear faster. The replacement of worn seals requires taking the apparatus out of service, removing the piston and replacing the seals.
Accordingly, there is a desire to continually improve the life of the piston seals to increase the time between service intervals, improving seal wear consistency over time.